Gateway GPRS Support Node, will need as a Full MVNO, ESP usually MNO hosts this and the SGSN obviously. In Full MVNO, MNO host still has SGSN.

GPRS

General Packet Radio Service, genereal term for 2G and beyond packet switched rather than circuit switched data in MVNO contracts especially

GPS

Global Positioning System

GSM

Global System for Mobile Communications, the de facto mobile standard, rival CDMA did not really take-off and in countries with both services, wholesale prices for GSM are higher, CDMA cheaper

HLR

Home Location Register, originally Ofcom decided an MVNO needs / should have one, however this was really unfounded and an HLR, although just a database, is a complex peice of equipment that an MNO is generally better runing for the MVNO

Subscriber Acquisition Costs, the cost of getting a user on your network - if you do not know this, and why you can do cheaper than your MNO for your chosen market, your meeting is likely to be rather short! basically equal cost of sale: Physical SIM, SIM pack, commissions, distribution costs, etc.

SGSN

Serving GPRS Support Node, the main mobile data "box" in the MNO network